where \(R\)  is the geometric mean of \(R_m\)  and \(R_n\) , \(R=2R_mR_n / (R_m+R_n)\) ,
\(\xi _{mn}\)  is the normal overlap, or penetration length, as depicted in Fig. REF a,
\(G\)  is the shear modulus and \(\nu \)  the Poisson ratio of the grains,
equal to \(0.5\)  for rubber [1]}.
<FIGURE>